1. Field of the Invention
The present invention relates to the field of still video imaging, and especially to exposure control apparatus for a still video camera of the type that produces a movie image for an electronic viewfinder in addition to a still image for recording.
2. Description Relative to the Prior Art
A still video camera, by virtue of its inherent video processing capability, is well suited for combination with an electronic viewfinder (see, for example, the camera disclosed in U.S. Pat. No. 4,456,931). This combination is particularly advantageous for previewing the picture as it will actually appear subsequent to video processing. Such a camera operates in two modes: a movie mode for producing a moving video image in the viewfinder and a still mode for producing and recording a still image. The movie mode is comparable to the operation of any motion picture video camera. In such a camera, the photosensitive region of an image sensor is continuously irradiated by scene light. Since the exposure time is fixed to accord with the video image rate (e.g., 1/30th second), the amount of light striking the image sensor is controlled by regulating the optical aperture presented to the incoming light with an adjustable diaphragm.
In the still mode, it is desirable that both the exposure time and the optical aperture are controlled so as to vary, respectively, the length of exposure and the intensity of the light striking the image sensor. For example, in order to obtain a distinct still image of an object in motion, it is necessary to shorten the exposure time so as to prevent image blur. In shortening the exposure time it is ordinarily necessary to open the aperture so as to maintain the same total exposure. On the other hand, in order to obtain a distinct still image over a broad distance range, it is necessary to close down the aperture so as to extend the depth of field. Then the exposure time would have to be lengthened accordingly. The adjustments are thus interactive, that is, exposure time adjustments require corresponding aperture adjustments, and vice-versa, in order to maintain a correct exposure value.
Since a common image sensor is the signal source for both modes, one mode excludes the other, that is, the movie mode can only precede, or follow, the still mode. This fact of operation leads to the conventional sequence of exposure control. For instance, in U.S. Pat. No. 4,456,931, an exposure operation circuit is disclosed that depends upon the output of a light measuring element for calculating an appropriate aperture value for an electronic display on a monitor. In this manner, a properly exposed video movie image is previewed prior to a still exposure. When a still exposure is called for, the exposure operation circuit interrupts the movie calculations and calculates an appropriate still aperture value from the brightness of the object sensed by the light measuring element and from an exposure time that is, e.g., manually selected.
In making the transition from movie to still mode, time must be allocated to moving the diaphragm from the movie aperture value to the still aperture value. Determining the still aperture value, moreover, involves a further cycle of the exposure operation circuit before the diaphragm can begin moving, i.e., a special signal integration period for measuring scene brightness and determining the proper still aperture and/or exposure time. While the time for measuring brightness is short . . . an integration can take a few milliseconds . . . every fractional second saved is critical at this stage. Once the exposure release has been actuated, the still image actually anticipated is already in the past. Thus it is important to get the exposure under way without delay. In the aforementioned U.S. Pat. No. 4,456,931, the exposure operation circuit is busy with the operation of the viewfinder up until the still exposure is triggered. Only then does it turn to the determination of the required aperture for the still exposure. The time thereby lost may critically affect the usefulness of the camera in many picture taking occasions, such as "snapshot" or "candid" situations where picture content is rapidly changing.